High voltage start-up circuit and method therefor

ABSTRACT

The present invention relates to a switchmode power supply (SMPS) high voltage start-up integrated circuit (IC) with output voltage sensing and output current limiting. The high voltage start-up IC allows low voltage pulse width modulated (PWM) controller ICs to operate directly off rectified AC lines of up to 450 VDC. The high voltage start-up IC allows PWM controller ICs to start-up with a start threshold larger than its operating voltage. The output voltage is monitored internally so that the internal high voltage switch turns off when the output voltage decreases below an internally set trip point voltage (V off ) The internal high voltage switch remains off and an external auxiliary voltage is generated and applied to the output voltage. If the output voltage falls below a lower set trip point voltage (V reset ) the internal high voltage switch turns back on. This allows the start-up circuit to reset itself when the PWM controller IC&#39;s auxiliary voltage does not power up properly.

RELATED APPLICATIONS

This U.S. patent application is a continuation of U.S. patentapplication entitled "HIGH VOLTAGE START-UP CIRCUIT AND METHODTHEREFOR," Ser. No. 08/490,593, filed Jun. 15, 1995, in the name of thesame Applicant as this U.S. patent application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to switchmode power supplies (SMPS)and, more specifically, to a start-up integrated circuit (IC) for a SMPSoperating from high input voltages ranging from 40 V to 450 V.

2. Description of the Prior Art

An off-line switchmode power supply (SMPS) accepts a rectified AC inputvoltage and converts and regulates it to various desired output DCvoltages. Common AC voltage sources are nominally 90 VAC, 120 VAC, and240 VAC. The conversion and regulation is accomplished by a pulse widthmodulated (PWM) controller integrated circuit (IC). The majority of PWMcontroller ICs will operate with a maximum input voltage of 30 V only.External circuitry is therefore required for the SMPS to operatedirectly off the rectified AC line. The external circuit is called thestart-up circuit. The start-up circuit is only used during start-up ofthe SMPS. Once the SMPS is started, an auxiliary voltage is generatedand is used to power up the PWM controller IC. A typical power resistordiscrete implementation of the start-up circuit uses a power resistor, acapacitor, and a zener diode.

PWM controller ICs typically have 2 current ratings: start-up currentand operating current. Start-up currents are typically less than 1.0 mAwhereas the operating currents are typically less than 17 mA. When theSMPS is powered up, the PWM controller IC will draw no more than 1.0 mAwhen its V_(cc) voltage is below its start threshold voltage. A typicalstart threshold voltage is 16 V. Once the V_(cc) voltage becomes greaterthan the start threshold voltage, the PWM controller IC will typicallydraw 14 mA. The power resistor is selected such that it will provide thecharging current for the external capacitor and the PWM controller IC'sstart-up circuit. The operating current is supplied by the capacitoracross V_(cc). The PWM controller IC will continue to operate until itsV_(cc) voltage falls below the minimum operating voltage rating,typically 10 V. An auxiliary voltage is applied to the V_(cc) of the PWMcontroller IC. It is desirable to have the auxiliary voltage to begreater than 10 V before the V_(cc) capacitor discharges below theminimum operating voltage. When the auxiliary voltage is greater than 10V, it will provide the required operating current for the PWM controllerIC.

The problem with the power resistor approach is that power iscontinuously being dissipated by the power resistor after start-up. Thepower dissipation will be worst if the same design is used for both 120VAC and 240 VAC operation. The high voltage start-up circuit of thepresent invention solves this problem by dissipating very little powerafter start-up. The start-up circuit monitors the output voltage anddetermines whether the SMPS has started up. After start-up, it turns offits internal high voltage input line to minimize power dissipation.Power is therefore only dissipated during start-up. This allows thestart-up IC to be packaged in a cost effective three (3) terminalpackage.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, it is anobject of this invention to provide an improved high voltage start-upcircuit for a switchmode power supply and method therefor.

It is another object of the present invention to provide an improvestart-up circuit and method therefor that will allow low voltage PWMcontroller ICs to be operated at considerably higher DC voltages.

It is still another object of the present invention to provide animproved start-up circuit and method therefor that will allow the PWMcontroller IC to be operated at a voltage below that of its thresholdvoltage after start-up.

It is still another object of the present invention to provide animproved start-up circuit and method therefor that allows the start-upcircuit's high voltage input current to be turned off with a voltagevalue less than that of the start threshold voltage.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with one embodiment of the present invention, a highvoltage start-up circuit for a switchmode power supply is disclosed. Thehigh voltage start-up circuit is comprised of switching means forturning off and on a high voltage input line of the start-up circuit andfor limiting a current flow from the high voltage input line to anoutput of the start-up circuit. A first comparator means is coupled tothe switching means for sending a signal to turn on the switching meanswhen an output voltage of the start-up circuit falls below a first setvalue. A second comparator means is coupled to the switching means forsending a signal to turn off the switching means when the output voltagefalls below a second set value.

In accordance with another embodiment of the present invention, a methodfor providing a high voltage start-up circuit for a switchmode powersupply is disclosed. The method is comprised of the steps of: providingswitching means for turning off and on a high voltage input line of thestart-up circuit and for limiting a current flow from the high voltageinput line to an output of the start-up circuit; providing firstcomparator means coupled to the switching means for sending a signal toturn on the switching means when an output voltage of the start-upcircuit falls below a first set value; and providing second comparatormeans coupled to the switching means for sending a signal to turn offthe switching means when the output voltage falls below a second setvalue.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following, more particular,description of the preferred embodiments of the invention, asillustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of the high voltage start-upcircuit of the present invention.

FIG. 2 is a simplified block diagram of the high voltage start-upcircuit of FIG. 1 integrated into a switchmode power supply.

FIG. 3 shows voltage and current timing diagrams for the high voltagestart-up circuit as implemented in a switchmode power supply as depictedin FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the high voltage start-up circuit 10 of the presentinvention is shown. When V_(out) is typically less than 7.0 V, thecomparator 12 will sense that V_(out) is less than 7.0 V. Thecomparator's 12 output will then be at a logic high state therebyresetting the D flip flop 14. The D flip flop's 14 output (Q) will be atlogic zero keeping the transistor 16 off.

V_(out) will continue to increase at a current limit set by the internalconstant current source 18, typically 3.0 mA. When V_(out) reaches above13.5 V, an output from a comparator 20 will switch from a logic high toa logic low. The D flip flop's 14 output does not change state since itsclock input is designed to trigger on a rising edge. V_(out) will stopincreasing once it reaches a maximum voltage V_(max) which is equal tothe voltage across the zener diode 22 minus the threshold voltage of thetransistor 24. The zener diode 22 is biased by a resistor 26. Theinternal voltage reference (V_(ref)) 28, and the resistor dividers 30,32, and 34 set the V_(out) trip points for the comparators 12 and 20. Inthe preferred embodiment of the present invention, the internal voltagereference 28 and the resistor dividers 30, 32, and 34 are set to haveV_(out) trip points of about 7.0 V and 13.5 V for the comparators 12 and20.

V_(out) will start to decrease when it is loaded down with an externalload greater than the internal constant current source 18. When V_(out)falls below 13.5 V, the output of the comparator 20 will switch from alogic low to a logic high. The output of the comparator 20 will clock ina logic one (1) into the D flip flop 14 causing the D flip flop's 14output to a logic high. Transistor 16 will then turn on pulling the gateof the transistor 24 to ground thereby turning off the transistor 24.

Referring to FIG. 2, a configuration of a SMPS 40 using the high voltagestart-up circuit 10 of the present invention. The high voltage start-upcircuit's 10 V_(out) terminal is connected to the V_(cc) line of thepulse width modulated (PWM) controller IC 42. An auxiliary winding 44Aon the transformer 44 is used to generate a V_(cc) voltage to power thePWM controller IC 42 after start-up. The auxiliary winding 44A isconnected to the V_(cc) line via diodes 46 and 48.

The diode 48 and the capacitor 50 are used in applications where theauxiliary voltage must use a large capacitance value for capacitor 52.Without the diode 48 and the capacitor 50, a large amount of time willbe required to charge the capacitor 52 since the output is currentlimited. The use of the diode 48 will block the flow of current goinginto the capacitor 52. The capacitor 50 must be at a much lower valuethan that of capacitor 52. This will allow the power supply to start-upfaster.

During power up, the high voltage start-up circuit 10 will start tocharge the capacitor 50 on the PWM controller IC 42. The V_(cc) voltagewill begin to rise. When V_(cc) is between 0 and 7 V, a reset signal isgenerated clearing an internal latch. Once the V_(cc) voltage risesabove 7 V, the reset signal is disabled. The V_(cc) voltage willcontinue to rise until it reaches the PWM controller IC's 42 startthreshold voltage, typically 16 V.

The PWM controller IC 42 will typically draw 0.5 mA from its V_(cc) lineduring start-up. Once V_(cc) reaches typically 16 V, the PWM controllerIC 42 will draw 14 mA and is considered to be in its operating mode.Since the start-up circuit can only supply a maximum current of 2 to 4mA, the PWM controller IC 42 will start discharging the capacitor 50.When in its operating mode, the PWM controller IC 42 can be operatedwith a minimum V_(cc) voltage of typically 10 V.

Once V_(cc) falls below 13.5 V, a latch in the high voltage start-upcircuit 10 will clock in a logic high. This turns off a high voltageinput line of the high voltage start-up circuit 10. At this point, thereis no current flowing from the high voltage input line on the highvoltage start-up circuit 10. V_(cc) will continue to decrease until thevoltage generated from the auxiliary winding reaches typically 10 V.

If the SMPS 40 is properly designed, the auxiliary voltage will supplythe required V_(cc) current for the PWM controller IC 42. If for somereason the auxiliary voltage does not reach 10 V, the V_(cc) voltagewill continue to decrease. When V_(cc) falls below the PWM controllerIC's 42 minimum operating voltage, the PWM controller IC will go backinto its start-up mode and will draw only 0.5 mA. Since the start-upcircuit is still off, the V_(cc) voltage will continue to decrease butat a much slower rate. When V_(cc) falls below 7 V, the high voltagestart-up circuit 10 will generate a reset signal and clears the internallatch thereby turning back on the high voltage line.

A switching transistor 54 is coupled to the transformer 44 and the PWMcontroller IC 42. The switching transistor 54 is used to charge theprimary of the transformer 44. The PWM controller IC will vary its pulsewidth depending on what the input voltage and output loads are tomaintain voltage regulation.

Referring to FIG. 3, voltage and current timing diagrams for the SMPS 40depicted in FIG. 2 are shown. Phase I is when the SMPS 40 is atstart-up. Phase II is when the PWM controller IC is operating but theSMPS 40 output has not settled. Phase III is when the SMPS output hassettled.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in form,and details may be made therein without departing from the spirit andscope of the invention.

I claim:
 1. A high voltage start-up circuit for a switchmode powersupply comprising, in combination:switching means for turning off and ona high voltage input line of said start-up circuit and for limiting acurrent flow from said high voltage input line to an output of saidstart-up circuit; comparator means coupled to said switching means forsending a signal to turn on said switching means when an output voltageof said start-up circuit falls below a first set value and for sending asignal to turn off said switching means when said output voltage fallsbelow a second set value.
 2. A high voltage start-up circuit inaccordance with claim 1 wherein said comparator means comprises:firstcomparator means coupled to said switching means for sending a signal toturn on said switching means when an output voltage of said start-upcircuit falls below a first set value; and second comparator meanscoupled to said switching means for sending a signal to turn off saidswitching means when said output voltage falls below a second set value.3. A high voltage start-up circuit in accordance with claim 2 furthercomprising trip point setting means coupled to said first and secondcomparator means for setting a value of said first set value and forsetting a value of said second set value.
 4. A high voltage start-upcircuit in accordance with claim 3 wherein said trip point setting meanscomprises:constant voltage source means coupled to said first and secondcomparator means for providing a reference voltage for said first andsecond comparator means; and resistor divider means coupled to saidfirst and second comparator means for setting said first and second setvalues.
 5. A high voltage start-up circuit in accordance with claim 4wherein said resistor divider means comprises:a first resistor coupledto said first comparator means; a second resistor serially coupled tosaid first resistor; and a third resistor serially coupled to saidsecond resistor.
 6. A high voltage start-up circuit in accordance withclaim 2 further comprising constant current source means coupled to saidswitching means for increasing a voltage output of said start-up circuitat a current limit set by said constant current source means until saidoutput voltage is loaded with an external source greater than saidconstant current source.
 7. A high voltage start-up circuit inaccordance with claim 2 wherein said switching means comprises:firsttransistor means coupled to said high voltage input line for turning offand on said high voltage input line and limiting said current flow fromsaid high voltage input line to an output of said start-up circuit;second transistor means coupled to said first transistor means forsending a signal to said first transistor means to turn off and on saidfirst transistor means; and D flip flop means coupled to said first andsecond comparator means and to said second transistor means for sendinga signal to said second transistor means to turn off and on said firsttransistor means.
 8. A method of providing a high voltage start-upcircuit for a switchmode power supply comprising the steps of:providingswitching means for turning off and on a high voltage input line of saidstart-up circuit and for limiting a current flow from said high voltageinput line to an output of said start-up circuit; providing comparatormeans coupled to said switching means for sending a signal to turn onsaid switching means when an output voltage of said start-up circuitfalls below a first set value and for sending a signal to turn off saidswitching means when said output voltage falls below a second set value.9. The method of claim 8 wherein said step of providing comparator meansfurther comprises the steps of:providing first comparator means coupledto said switching means for sending a signal to turn on said switchingmeans when an output voltage of said start-up circuit falls below afirst set value; and providing second comparator means coupled to saidswitching means for sending a signal to turn off said switching meanswhen said output voltage falls below a second set value.
 10. The methodof claim 9 further comprising the step of providing trip point settingmeans coupled to said first and second comparator means for setting avalue of said first set value and for setting a value of said second setvalue.
 11. The method of claim 10 wherein said step of providing trippoint setting means further comprises the steps of:providing constantvoltage source means coupled to said first and second comparator meansfor providing a reference voltage for said first and second comparatormeans; and providing resistor divider means coupled to said first andsecond comparator means for setting said first and second set values.12. The method claim 11 wherein said step of providing resistor dividermeans further comprises the steps of:providing a first resistor coupledto said first comparator means; providing a second resistor seriallycoupled to said first resistor; and providing a third resistor seriallycoupled to said second resistor.
 13. The method of claim 9 furthercomprising the step of providing constant current source means coupledto said switching means for increasing a voltage output of said start-upcircuit at a current limit set by said constant current source meansuntil said output voltage is loaded with an external source greater thansaid constant current source.
 14. The method of claim 9 wherein saidstep of providing switching means further comprises the stepsof:providing first transistor means coupled to said high voltage inputline for turning off and on said high voltage input line and limitingsaid current flow from said high voltage input line to an output of saidstart-up circuit; providing second transistor means coupled to saidfirst transistor means for sending a signal to said first transistormeans to turn off and on said first transistor means; and providing Dflip flop means coupled to said first and second comparator means and tosaid second transistor means for sending a signal to said secondtransistor means to turn off and on said first transistor means.
 15. Ahigh voltage start-up circuit for a switchmode power supply comprising,in combination:high voltage input line for supplying power to saidstart-up circuit; switching means coupled to said high voltage inputline for turning off said high voltage input line by sensing a decreasein an output voltage of said start-up circuit; trip point setting meanscoupled to said switching means for resetting and turning on said highvoltage input line; and current limiting means coupled to said highvoltage input line for limiting current flow from said high voltageinput line to an output of said start-up circuit.